REVISITING PRECAMBRIAN CARBONATE MUD PRODUCTION: EVIDENCE THAT ABRASION PLAYED A KEY ROLE (Invited Presentation)

Beginning with the evolution of calcifying algae, carbonate-biomineralizing eukaryotes have exerted a considerable influence on the carbonate rock record via the production of carbonate particles—both mud and allochems—and framework elements like reefs. Prior to the advent of biomineralization, carbonate deposition is thought to be dominated by chemical and microbial precipitation, presumably aided by a higher carbonate saturation state in seawater. For example, direct precipitation from carbonate-supersaturated seawater is hypothesized to be the primary mechanism for producing carbonate mud in Precambrian time, even though our understanding of this mechanism on modern carbonate platforms remains incomplete and controversial. We hypothesized that physical abrasion of carbonate grains was responsible for a significant fraction of carbonate mud production during Precambrian time, relaxing this reliance on seawater carbonate chemistry. We designed a series of wet abrasion mill experiments to quantify the rate of mud production via physical abrasion of carbonate sand grains during transport. These experimental rates were more rapid than direct precipitation in the water column, demonstrating that abrasion of carbonate sand was likely an important source of carbonate mud throughout Earth history, particularly during Precambrian time prior to algal mud production. Furthermore, we predicted that grainy carbonate sediments transported in bed load or suspended load in shallow environments should be the most significant sources for producing carbonate mud by abrasion. To evaluate whether the carbonate rock record is consistent with this mechanism, we compared temporal trends in carbonate facies using a database of high-resolution facies trends from the Precambrian and Cambrian. We examined the abundance of mudstones—a proxy for mud production, grainstones in shallow marine environments—the sand-sized material with potential to interact relatively frequently with currents, and other precipitated facies that are sensitive to the trajectory of carbonate saturation state. We used this facies data to build a Precambrian carbonate mud budget, evaluating the contributions of seawater precipitation vs. abrasion over time.